1. Field of the Invention
The present invention relates to an antenna apparatus. More particularly, the invention relates to a multiband antenna apparatus capable of transmitting and/or receiving radio waves of different frequency bands.
2. Description of the Related Art
The recent popularization of portable communication terminal apparatuses, typically cellular phones, raises a problem stringent frequency bands to be used in the portable communication terminal apparatuses. This requires a multiband antenna apparatus like a dual-band type, which can transmit/receive radio waves of different frequency bands. The multiband configuration of antenna apparatuses results in increases in the size and quantity of antenna apparatuses. To make cellular phones smaller and lighter, however, more compact antenna apparatuses are demanded.
A chip antenna is frequently used as a compact antenna.
FIG. 7 shows a conventional antenna apparatus 6. A first antenna element 62 and a second antenna element 63 are formed on a flat face portion 611 of a support base member 61. The first antenna element 62 and the second antenna element 63 are arranged with a predetermined clearance between distal ends thereof.
The antenna elements are each adjusted to have an antenna characteristic, for example, as shown in FIG. 8. In FIG. 8, the horizontal axis represents the resonance frequency of each antenna element, and the vertical axis represents the value of a VSWR (Voltage Standing Wave Ratio) at a power feeding end of each antenna element. It is preferable that the VSWR of the frequency band to which each antenna element is made to correspond should be 3.0 or less. With the preference in mind, the antenna characteristic shown in FIG. 8 indicates that one of the first antenna element 62 and the second antenna element 63 corresponds to a frequency band near 800 MHz, while the other antenna element corresponds to a frequency band near 2 GHz.
There is an antenna having a plurality of antenna elements to transmit/receive radio waves of a single frequency band, not plural frequency bands.
For example, Japanese Patent No. 3514305 and Japanese Patent No. 3551368 describe techniques of simultaneously transmitting/receiving two linear polarized components of a radio wave of a single frequency band whose polarization planes are orthogonal to each other.
Specifically, Japanese Patent No. 3514305 discloses an antenna including elements and slots for irradiating two linear polarized components whose polarization planes are orthogonal to each other.
Japanese Patent No. 3551368 discloses an antenna including an antenna element and a power feed line which irradiate a horizontal polarized component and further including a conductive element which irradiates a vertical polarized component.
In the conventional antenna apparatus shown in FIG. 7, the end faces of the distal ends of the two antenna elements are arranged close to each other and facing each other. Accordingly, the distal ends of the two antenna elements show the characteristic of a capacitor.
A capacitance C stored between the distal ends of the two antenna elements is derived from an equation 1 below.C=ε0·εr·(S/d)  (1)where ε0 is the dielectric constant of vacuum, εr is the relative permittivity of the support base member (or air), S is the area of each opposing distal end, and d is the distance between the opposing distal ends.
It is apparent from the equation 1 that the value of the capacitance C increases according to the area S of the opposing distal end. According to the conventional antenna apparatus, the area S of the opposing distal end is equivalent to the cross-sectional area of the distal end of the antenna element. Accordingly, the capacitance C according to the size of the cross-sectional area is stored between the distal ends of the two antenna elements. The electric coupling of the antenna elements originating from the storage of the capacitance C is likely to degrade the antenna performance of each antenna element, such as the antenna gain and reception sensitivity.
Even in a case where each antenna element is so adjusted as to correspond to the frequency band of radio waves to be transmitted/received, the electric coupling of the antenna elements, if strong, would cause interference between the antenna elements. Specifically, when one antenna element is adjusted to correspond to a predetermined frequency band, as shown in FIG. 9, the frequency band to which the other antenna element can be adapted deflects according to the amount of the adjustment. When multiple antenna elements interfere with one another, the adjustment of the frequency bands becomes troublesome.
The techniques described in Japanese Patent No. 3514305 and Japanese Patent No. 3551368 are directed to polarization of polarization planes of radio waves to be irradiated from each antenna element. The techniques are premised on transmission/reception of radio waves of the same frequency band. Therefore, the techniques described in Japanese Patent No. 3514305 and Japanese Patent No. 3551368 cannot overcome the foregoing problem of a plurality of which transmit/receive radio waves of different frequency bands.